Shell molding

Shell molding, also known as shell-mold casting,[1] is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision.[2]

Examples of shell molded items include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores.

Contents

Process

The process of creating a shell mold consists of six steps:[2][3]

  1. Fine silica sand that is covered in a thin (3–6%) thermosetting phenolic resin and liquid catalyst is dumped, blown, or shot onto a hot pattern. The pattern is usually made from cast iron and is heated to 230 to 315 °C (450 to 600 °F). The sand is allowed to sit on the pattern for a few minutes to allow the sand to partially cure.
  2. The pattern and sand are then inverted so the excess sand drops free of the pattern, leaving just the "shell". Depending on the time and temperature of the pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).
  3. The pattern and shell together are placed in an oven to finish curing the sand. The shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).
  4. The hardened shell is then stripped from the pattern.
  5. Two or more shells are then combined, via clamping or gluing using a thermoset adhesive, to form a mold. This finished mold can then be used immediately or stored almost indefinitely.
  6. For casting the shell mold is placed inside a flask and surrounded with shot, sand, or gravel to reinforce the shell.[4]

The machine that is used for this process is called a shell molding machine. It heats the pattern, applies the sand mixture, and bakes the shell.

Details

Setup and production of shell mold patterns takes weeks, after which an output of 5–50 pieces/hr-mold is attainable. Common materials include cast iron, aluminum and copper alloys.[1] Aluminum and magnesium products average about 13.5 kg (30 lb) as a normal limit, but it is possible to cast items in the 45–90 kg (100–200 lb) range. The small end of the limit is 30 g (1 oz). Depending on the material, the thinnest cross-section castable is 1.5 to 6 mm (0.06 to 0.24 in). The minimum draft is 0.25 to 0.5 degrees.[1]

Typical tolerances are 0.005 mm/mm or in/in because the sand compound is designed to barely shrink and a metal pattern is used. The cast surface finish is 0.3–4.0 micrometers (50–150 μin) because a finer sand is used. The resin also assists in forming a very smooth surface. The process, in general, produces very consistent castings from one casting to the next.[3]

The sand-resin mix can be recycled by burning off the resin at high temperatures.[4]

Advantages and disadvantages

One of the greatest advantages of this process is that it can be completely automated for mass production.[2] The high productivity, low labor costs, good surface finishes, and precision of the process can more than pay for itself if it reduces machining costs. There are also few problems due to gases, because of the absence of moisture in the shell, and the little gas that is still present easily escapes through the thin shell. When the metal is poured some of the resin binder burns out on the surface of the shell, which makes shaking out easy.[1][3]

One disadvantage is that the gating system must be part of the pattern because the entire mold is formed from the pattern, which can be expensive. Another is the resin for the sand is expensive, however not much is required because only a shell is being formed.[3]

References

Notes

  1. ^ a b c d Degarmo 2003, p. 310.
  2. ^ a b c Degarmo 2003, p. 308.
  3. ^ a b c d Degarmo 2003, p. 309.
  4. ^ a b Todd, Allen & Alting 1994, p. 267.

Bibliography

 
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